Examples of device which can obtain microscopic surface information of a sample by bringing a probe into proximity of the sample may be scanning probe microscope. Known examples of conventional scanning probe microscope include: an atomic force microscope (hereinafter, referred to as “AFM”.) which detects the interaction force between the sample and the probe by using a cantilever; and a magnetic force microscope (hereinafter, referred to as “MFM”.) wherein a magnetic material is used as a probe of an atomic force microscope (see, for example, Patent documents 1 and 2).
In these scanning probe microscopes, it is the so-called “AC mode” to measure force gradient of such as electric forces, magnetic forces, and van der Waals forces caused by bringing the probe, which is provided at a proximity of the free end, into the proximity of the sample when a piezoelectric element for exciting cantilever and the like are excited by alternating voltage. When the cantilever is excited with a predetermined oscillation frequency, it is as if spring constant of the cantilever is changed by the force gradient of the sample. So, in the AC mode, by using the above property, fluctuation of the resonance characteristic of the cantilever is detected to measure the shape of sample's surface, electric forces, and magnetic forces. In other words, if an effective spring constant of the cantilever changes, resonance frequencies of the cantilever also changes; if the cantilever is excited at a oscillation frequency, oscillation amplitude and phase of the cantilever (probe) change.
In the AFM which adopts AC mode, the probe is formed at a tip of cantilever of which spring constant is 0.01 N/m to dozens of Newton per meter and of which resonance frequencies is several to several hundreds of kilohertz. The cantilever is fixed to an excitation actuator comprising a piezoelectric element and arranged so that the cantilever faces a sample surface supported on a fine-motion element, namely xy-plane. When the probe and sample relatively move, the probe scans the sample surface.
During the scanning, driving voltage which enables to oscillate the probe with a predetermined amplitude in a perpendicular direction to the sample surface is applied to the excitation actuator with a frequency near the resonance frequencies of the cantilever. The fine-motion element comprising the piezoelectric element to support the sample is controlled at an accuracy of 0.1 nm or less so that the oscillation amplitude of the probe is constant; and the sample is relatively moved in the perpendicular direction to the sample surface, namely z-direction. Consequently, the tip of the probe traces on a curved surface which reflects the surface shape of the sample.
Therefore, when the position of the tip of the probe on the xy-plane and the position of the same in the z-direction is obtained by applying the voltage to the fine-motion element and recording it, it is possible to obtain an AFM image showing fine recess and protrusion of the sample surface.
At this time, when using a magnetic materials (for example, CoCr, FrPt, and permalloy) as the probe and using a magnetic device (for example, magnetic tape, hard disk media, magnetic head, and magneto-optical disk media) as the sample, the force gradient which affects the image of AFM is not only short-range forces which work at the vicinity of the sample surface, for example, van der Waals forces, but also magnetic force as a long-range force.
Accordingly, when the distance between the probe and the sample is short, image of the topography of the sample surface is mainly observed; when the distance between the probe and the sample is long, image of the magnetic force is mainly observed. The distance between the probe and the sample during scanning can be adjusted by keeping decreasing rate of the oscillation amplitude constant. For instant, when the cantilever is excited at a frequency slightly higher than the resonance frequencies of the cantilever, if the probe is brought into proximity of the sample, resonance frequencies of the cantilever decreases because of the attracting force between the probe and the sample, thereby oscillation amplitude of the cantilever which is excited at a constant excitation frequency decreases. Hence, when the decreasing rate increases, the probe moves closer towards the sample. The obtained image is the image with constant force gradient. As it were, the image with constant force gradient, which is obtained by measuring in a manner that decreasing rate of oscillation amplitude of the cantilever is constant under a condition where the distance between the probe and the sample is short, is a profile of surface topography.
On the other hand, the image of constant force gradient, which is obtained by measuring in a manner that decreasing rate of oscillation amplitude of the cantilever is constant under a condition where the distance between the probe and the sample is long, is an image with constant magnetic force gradient. However, since the distance between the probe and the sample is not constant, the image includes not only magnetic information but also information of surface topography. When evaluating the magnetic recording media and so on, surface topography and magnetic field information have not been separated.
A method is proposed to measure magnetic force gradient under a condition that distance between the probe and the sample is constant, the method comprises the steps of: during scanning with the probe, firstly, intermittently bringing the probe into contact with the sample under a condition that decreasing rate of the oscillation amplitude of the cantilever is large and measuring the surface topography of a sample along a scanning line; then, making the probe apart from a sample surface with a certain height (probe-sample distance) so that the magnetic force works as the main force from the sample at the same location in xy-plane, oscillating the cantilever and recording the oscillation amplitude or phase. When employing the method, it is possible to obtain a magnetic force image at the same position as the profile of surface topography. Therefore, with magnetic recording media and so on, it is also possible to know the effect of thin-film texture related to the surface topography on the magnetic domain structure.
In recent years, with high-density magnetic recording, to evaluate microscopic magnetic domain structure of the recording media, improvement of spatial resolution of MFM has been required. Moreover, with faster speed of the magnetic storage devices, it is important to evaluate frequency property of the high-frequency magnetic field generated from the magnetic device and high-frequency response of the soft magnetic materials used for magnetic devices. To improve the spatial resolution of MFM, it is necessary to improve detection sensitivity of the force gradient; it can be effectively attained by improving mechanical resonance characteristic of the cantilever. The resonance characteristic of the cantilever depends on measurement atmosphere. In a vacuum atmosphere that air viscosity is small, compared with air atmosphere, the resonance characteristics enhances significantly.
The resonance characteristic can be evaluated by a value of performance factor “Q” of resonance. By the resonance, the detection sensitivity increases about Q-fold. The Q-value under an air atmosphere is several hundred; however Q-value under a vacuum atmosphere increases up to several thousand to several million. Therefore, by exciting cantilever near the resonance frequencies having a large Q-value with an excitation actuator comprising piezoelectric element, spatial resolution can be improved. With respect to the measurement of high-frequency magnetic field by using MFM, frequency component of the high-frequency magnetic field needs to include the value near the resonance frequencies (f0) of the cantilever or high-order component of the resonance frequencies (n×f0: n is a positive integer). Examples measured in the past may be an alternating magnetic field of a single frequency near the resonance frequencies of the cantilever and an alternating magnetic field which has a resonance frequencies component of the probe and of which amplitude is modulated. Conventionally, measurement of these specified frequencies, except for those in the alternating magnetic field, has been difficult (see Non-Patent Document 1.).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 08-122341    Patent Document 2: JP-A No. 2003-065935    Non-Patent Document 1: Wei. J.-D, and three others, “Observation of stray fields from hard-disk writer poles up 2 GHz”, IEEE Transactions on Magnetics, June 2007, Vol. 43, No. 6, pp. 2205-2207.    Non-Patent Document 2: Masayuki ABE, Yoichiro TANAKA, “A Study of High-Frequency Characteristics of Write Heads With the AC-Phase High-Frequency Magnetic Force Microscope”, IEEE Transactions on Magnetics, January 2002, Vol. 38, pp. 45-49.